1. Field of the Invention
The present invention relates to antenna devices suitable for being incorporated into in-vehicle telecommunication systems and the like.
2. Description of the Related Art
As shown in FIG. 5, an antenna device having a meandering radiating conductor patterned on a substrate is known as a compact antenna with a reduced height for being incorporated into an in-vehicle telecommunication system and the like (see, e.g., Japanese Unexamined Patent Application Publication No. 2000-349532 (in particular, pages 3 to 4, FIG. 1)).
In an antenna device 1 shown in FIG. 5, a meandering radiating conductor 3 made of copper foil is formed on a surface of a dielectric substrate 2 that is placed upright on a ground conductor 4, and high-frequency power is supplied to the lower end of the radiating conductor 3 via a power feeder such as a coaxial cable. As compared to the height of a radiating conductor formed in a straight line and having the same electrical length, the height of the meandering radiating conductor 3 is significantly lower, and thus use of the meanderline structure is advantageous in reducing the height of the antenna as a whole.
As shown in FIG. 6, moreover, an antenna device with a radiating conductor including two different pitches of meandering lines joined together and formed on a substrate surface has been used as a compact antenna that can send and receive signal waves of two frequency bands (see, e.g., Japanese Unexamined Patent Application Publication No. 2001-68917 (in particular, pages 3 to 4, FIG. 1)).
In a dual-band antenna device 5 shown in FIG. 6, a radiating conductor 8 made of copper foil is patterned on a surface of a dielectric substrate 7 that is placed upright on a ground conductor 6. The radiating conductor 8 is a combination of a first radiating conductor 8a meandering from the side adjacent to a feeding point with a relatively wide pitch, and a second radiating conductor 8b meandering from the end of the first radiating conductor 8a with a relatively narrow pitch. Supply of power of a first high-frequency to the feeding point of the radiating conductor 8 via a power feeder such as a coaxial cable allows the entire radiating conductor 8, which extends from the first radiating conductor 8a to the second radiating conductor 8b, to resonate at a first frequency f1. However, supply of power of a second high-frequency to the feeding point allows only the first radiating conductor 8a to resonate at a second frequency f2 that is higher than the first frequency f1. Since a meandering line with a narrow pitch (the second radiating conductor 8b) tends to impair the flow of a high-frequency current with a higher frequency, the second frequency f2 can allow only the first radiating conductor 8a to function as a radiating element.
In the above-described antenna device 1 and the antenna device 5 that are known, excessively narrow meandering pitches of the radiating conductor 3 and the radiating conductor 8 tend to cause higher mode resonances. A possible approach to reducing the antenna height, in this case, is to narrow the widths of the radiating conductor 3 and the radiating conductor 8, but their excessively narrow widths result in reduction in gain and narrowing of the resonant frequency band. In the antenna device 1 and the antenna device 5, therefore, it is difficult to reduce the antenna height while maintaining a sufficient gain and bandwidth.
Reducing the height of the entire antenna is particularly difficult in the dual-band antenna device 5, because in the radiating conductor 8a and the radiating conductor 8b the two different meandering pitches are connected in series. This inevitably increases the length of the radiating conductor 8.